BOSTON—Safe to say that, in the long history of pro basketball here in Boston’s North End, there have not been many afternoons quite as strange as what the Celtics underwent when they played the Miami Heat at TD Garden on Sunday.

The Celtics, playing the part of gritty underdogs, won a double-overtime thriller, 100-98, in which there were 19 lead changes and 20 ties and no team established a lead of more than eight points. But even though the Celtics finally snapped their six-game losing streak, while the Heat saw their four-game win streak broken, the result was well down on the list of events of significance on the day.

In the morning, word came that point guard Rajon Rondo had hyperextended his right knee in Friday’s loss to the Atlanta Hawks and would not play on Sunday. Then word trickled out that the Celtics were concerned that the injury was much worse than a hyperextension. Finally, Boston’s PR staff confirmed during the second half of the game that Rondo, in fact, had torn his ACL and would be out for the year.

That’s a devastating blow for a team that has looked to Rondo as its most consistent player, an All-Star starter who was averaging 13.8 points and a league-high 11.1 assists.

You know things are going bizarro when even LeBron James, one of Boston’s least-liked sporting villains, offers his sympathy. “I think it (stinks),” James said of the Rondo injury. “It’s terrible. As much as a competitor and as much as I’ve been a rival with Boston over the years, I never want to see anyone go down, and knowing the competitor that he is and knowing how talented he is, I think it’s terrible, not only for their team but for the league.”

Rondo thought he had a hamstring injury, but Celtics coach Doc Rivers said he was told before the game that Rondo likely had a bigger knee problem than expected. When the MRI confirmed that, Rivers’ heart sank. Rivers decided not to tell his team until after the game, heightening the overall weirdness of the coach’s day.

Additionally, it was the second straight game the Celtics played two overtimes.

“I was just trying to coach,” Rivers said. “I was going in to talk to the team, you know, 20, 24 minutes before the game and Rondo was still in the lineup. (Team physician Dr. Brian) McKeon grabbed me and said he’s out. And I said, ‘He went through our walkthrough.’ And he said, ‘No, he’s out.’”

Boston was sagging in the Eastern Conference even with Rondo healthy and playing at a high level—he had recorded back-to-back triple-doubles—and the Celtics seemed to be counting on a strong midseason surge to get themselves back to their usual perch among the conference elite. Starting the day, they were eighth in the East, 3 1/2 games behind the seventh-place Milwaukee Bucks. Behind them were the unlikely trio of the Toronto Raptors, Detroit Pistons and Philadelphia 76ers, all within five games.

The loss of Rondo might have been the only bit of news that could have shoved all of the other Celtics-Heat story lines to the side. There was the fact that the Heat are returning to Boston after winning the NBA Finals, and that their last time in this building—for Game 6 of the Eastern Conference finals—star forward LeBron James scored 45 points and had 15 rebounds, finally exorcising his Boston demons. James was pretty good again on Sunday, scoring 34 points on 14-for-31 shooting, with 16 rebounds and seven assists.

Coming into the game, just about all of the 18,624 in attendance were eager to get their first look at Ray Allen, who was making his first appearance in Boston since he bolted the Celtics, for whom he played five seasons, as a free agent last summer. There was much bitterness in these parts in the wake of that move; the Celtics offered Allen more money but Allen still chose to join Boston’s fiercest rival.

And he produced, scoring 21 points and grabbing five rebounds. Allen made a big shot with 25.1 seconds to play in regulation, a 3-pointer from the left corner that cut a four-point Boston lead to 85-84. He missed a potential game-tying 3 with 15 seconds to play from the same corner, but James picked up the slack by hitting a top-of-the-key 3 with 7.0 seconds to go to send the game into overtime.

Allen heard it from the crowd throughout the afternoon. “I didn’t know what to expect,” he said. “The one thing I was going to do is come into it and just focus on being prepared and getting the guys ready that were playing, that were starting the game. Early game is always tough regardless of the circumstances. I didn’t expect to get booed the whole time, throughout the game, that I touched the ball. That was interesting.”

It was in the second overtime that the bizarreness of the day came full circle. Just as the teams were set to tip off the period, Rondo came limping out of the tunnel. Not in a Willis Reed kind of way, however. He was wearing a black coat and black sweater, the sort of civilian attire he will need to get used to wearing. Making matters worse for the Celtics: As their best player was nursing a bum knee, the two guys brought in to replace Allen—Jason Terry and Courtney Lee—continued to look offensively inept. They combined to shoot 5-for-19 on the game, scoring 17 points. Leandro Barbosa, signed after training camp started and almost as an afterthought, was the difference-maker, with nine points.

The slumps of Terry and Lee, topped by the Rondo injury, don’t bode well for Rivers, but he said he expects someone on the roster to pick up the slack, and that playing without Rondo won’t doom the Celtics. “You can write the obituary,” Rivers said. “I’m not. You can go ahead, but I’m not. We won tonight, so the way I look at it is, we’re going to stay in there.”

To do that, though, Terry and Lee will not only have to help fill the void left by Allen—they’ve come up short thus far—but they’ll have to pitch in for Rondo, too. There was some inspiration in the Celtics’ play on Sunday, but eventually, they’ll need more than inspiration. They’ll need good guards.